1. Field of the Invention
The present invention relates to a reproduction apparatus that reproduces information from a recording media such as an optical disk, which has plural recording layers, and a focus jumping method for the reproduction apparatus.
2. Description of the Related Art
As an optical recording medium capable of recording or reproducing information optically, an optical disk is widely known. A semiconductor laser or the like is used as a light source to irradiate light beams condensed finely via a lens on the optical disk, whereby information is recorded in the optical disk or reproduced from the optical disk. As publicly known, a focus servo operation is performed in order to keep laser beams in a focused state with respect to a recording layer of the optical disk. The focus servo is performed by moving an object lens, which is held by a biaxial mechanism (a biaxial actuator) in an optical head, in a direction in which the object lens approaches and separates from the optical disk, that is, a focus direction, on the basis of a focus error signal.
In recent years, disks having plural recording layers, for example, a two-layer disk and a multilayer disk having three or more layers are developed as optical disks. In that case, in order to shift from a recording/reproduction operation in a certain recording layer to a recording/reproduction operation in another recording layer, focus jump is performed. For example, in order to shift from a state in which the focus servo is being applied in a first layer to a state in which the focus servo is applied in a second layer, focus jump movement of an object lens is performed.
Note that the focus jump is an operation for moving a recording layer serving as a focus position of a laser and is also referred to as layer jump.
JP-A-2002-269770, JP-A-2001-319344, JP-A-2002-279654, and JP-A-11-191222 disclose techniques for operations called focus jump or layer jump.
When the focus jump is performed, an object lens held by a biaxial mechanism is forcibly moved in a focus direction. In that case, a system using a kick pulse and a brake pulse as focus jump drive signals given to the biaxial mechanism has been implemented.
A focus jump operation, which is performed by giving the kick pulse and the brake pulse to the biaxial mechanism, will be explained with reference to FIG. 22.
(f) in FIG. 22 schematically shows a state in which laser beams are focused on a layer 0 of a disk. (g) in FIG. 22 schematically shows a state in which laser beams are focused on a layer 1. An example of focus jump for shifting focus from the state in (f) in FIG. 22 to the layer 1 as shown in (g) in FIG. 22 will be explained.
In the case of (f) in FIG. 22, an object lens 3a is held in a position for focusing laser beams on the layer 0 by a biaxial mechanism 3e. A kick signal is given to the biaxial mechanism 3e in this state to move the object lens 3a upward on the drawing. Thereafter, a brake signal is given to the biaxial mechanism 3e at required timing to decelerate the movement near a focus point of the layer 1 and turn on the focus servo at a certain point. Consequently, the focus jump for bringing laser beams into a state of focus on the layer 1 is completed as shown in (g) in FIG. 22.
(a) in FIG. 22 shows a focus error signal FE that is observed in such an operation.
The focus servo is on and applied to the layer 0 until a point t0 and the focus error signal FE is substantially at a zero level.
At the point t0, the focus servo is turned off. Then, a kick pulse shown in (e) in FIG. 22 is generated as a focus jump drive signal and an electric current corresponding to the kick pulse is fed to a focus coil of the biaxial mechanism 3e whereby the object lens 3a starts moving upward. Therefore, as the focus error signal FE, first, a waveform S0, which is half an S waveform, near a focus position of the layer 0 is observed after the point t0.
When the object lens 3a moves in that state, as the focus error signal FE, a quasi waveform Z may be generated because of, for example, influence of irregular reflection of an interlayer film. However, a waveform S1, which is a former half of the S waveform, is observed near a focus point of the layer 1. Therefore, it is possible to draw focus into the layer 1 if the focus servo is turned on at zero cross timing of the S waveform S1.
In order to perform this operation, first, an FcmpH slice level and an FcmpL slice level are set for the focus error signal FE.
It is possible to obtain an FcmpL signal in (c) in FIG. 22 by comparing the focus error signal FE and the FcmpL slice level. In addition, it is possible to obtain an FcmpH signal in (b) in FIG. 22 by comparing the focus error signal FE and the FcmpH slice level.
It can be confirmed that the movement of the object lens 3a is started if the FcmpL signal rises according to the S waveform S0 of the focus error signal FE after the kick pulse is given. Thus, the kick pulse is ended at rising edge timing of the FcmpL signal.
Thereafter, the object lens 3a continues the movement. When the FcmpH signal rises according to the S waveform S1 of the focus error signal FE, the object lens 3a reaches near the focus point of the layer 1. Thus, a brake pulse is given as a focus jump drive signal at that timing for a certain predetermined period. Then, the movement of the object lens 3a is decelerated. For example, if the FcmpH signal falls, it can be judged that the falling edge is near a zero cross point of the S waveform. Therefore, the focus jump is completed by turning on the focus servo at that timing.
Note that it is possible that the quasi waveform Z is generated and the FcmpH signal builds up before the S waveform S1. Thus, an FcmpH mask period in (d) in FIG. 22 is set only for a required period from the falling edge of the FcmpL signal such that the rising edge of the FcmpH signal due to the quasi waveform Z is not detected.